1. Technical Field of the Invention
The present invention relates to an image recording apparatus.
2. Description of Related Art
Image recording apparatuses such as an electrophotographic printer use an electrophotographic method. A photosensitive drum surface is uniformly charged by a charging device and light illuminates the charged surface to form an electrostatic latent image thereon. Then the electrostatic latent image is developed into a toner image by a developing device which deposits toner on the electrostatic latent image formed on the photosensitive drum. The toner image is then transferred to the print medium by a transferring device. The toner image transferred to the print medium is fixed by a fixing device.
FIG. 14 illustrates a general construction of a conventional image recording apparatus.
A charging roller 17 applies a negative voltage to the surface of a photosensitive drum 16 rotating in a direction shown by arrow A, so that the surface of the photosensitive drum 16 is negatively charged. An LED head 13 illuminates the surface of the photosensitive drum 16 to form an electrostatic latent image thereon.
Then, a developer 18 deposits negatively charged toner 10 to the photosensitive drum 16 to develop the electrostatic latent image into a toner image. When a print medium 37 is fed between the photosensitive drum 16 and a transfer roller 14, the transfer roller 14 receives a positive voltage so that the toner image is transferred to the print medium 37.
With the aforementioned conventional image recording apparatus, if the print medium 37 is a sheet made of an insulating material such as a transparent OHP (overhead projector) sheet, negative charges are apt to be stored on the insulating sheet. When the transparent sheet 37 passes between the photosensitive drum 16 and transfer roller 14, discharge takes place from the photosensitive drum 16 to the print medium 37. Therefore, a surface potential of the transparent medium 37 is negative.
The amount of negative charges increases since there is no path through which the negative charges stored on the surface of the transparent medium 37 can leak. As a result, the negative charges accumulates on the surface of the transparent medium 37 and the negative potential on the surface increases, being detrimental to effective transfer of the toner 10 to the print medium 37.
FIG. 15 illustrates the transfer process of the conventional image recording apparatus. Referring to FIG. 15, the width of the print medium 37 is narrower than the photosensitive drum 16 and transfer roller 14. When the high voltage is applied to the transfer roller 14, a transfer current Ia flows through the print medium 37 sandwiched between the transfer roller 14 and photosensitive drum 16. During the transferring process, an unwanted current Ib also flows directly from the transfer roller 14 to the photosensitive drum 16.
If the print medium 37 is thick, the impedance of the print medium 37 is high. Therefore, the transfer current Ia decreases and the unwanted current Ib increases by that amount with the result that the toner image 98 is transferred incompletely to the print medium. One way of increasing Ia is to increase the transfer voltage. However, changing the transfer voltage in accordance with the various widths of the print medium 37 is difficult and an increase in transfer voltage results in an increase in unwanted current Ib. An increased unwanted current Ib causes damages to the surface of the photosensitive drum 16.